The present invention relates to a method of preparing a pulp for digestion in a continuous process from a fiber-containing cellulose material.
U.S. Pat. No. 3 620 911 discloses a method of preparing a paper pulp from a fibrous nonwoody lignocellulose plant material having fibers and pith therein. After wet depithing the plant material and separating the pith from the fibers the resultant fiber slurry is partially dewatered and the fibers are then impregnated by adding an alkaline hydroxide solution to the fiber slurry, said impregnation being non-continuous and performed at atmospheric pressure. The mixture of impregnated fibers and alkaline hydroxide solution is then subjected to a digestion process.
Some types of plant material, such as bagasse, contain various types of fibre tissues which have different ability to absorb cooking liquor and hence different cooking requirements. The more reactive tissues require less chemicals for digestion but have the highest take-up of chemicals, i.e. they are the easiest for the cooking liquor to permeate. Conversely, the denser and harder tissues have the lowest degree of take-up and require more chemicals for digestion. The last-mentioned tissues, generally pertaining to the schlerenquima of the cane, provide the best fibers yielding the strongest pulp after digestion.
In commercial operation, bagasse fibers are cooked to the requirements of said denser tissues in order to reduce the amount of reject. Consequently, the more open and reactive tissues are digested for longer than is in fast necessary. More chemicals than theoretically needed are therefore used and the yield and final pulp quality are reduced. Such problems are associated with the method described in the above-mentioned U.S. Pat. No. 3,620,911 and also in U.S. Pat. No. 2,913,362 relating to a non-continuous method of producing cellulosic pulp.